530 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



surface being determined in each place very largely by the type of 

 substratum. 



2. In warm-temperate regions the marine benthic vegetation is 

 very various in different areas such as the Mediterranean and the 

 seas of southernmost Africa and Australia, the differences being sup- 

 posedly due largely to historical causes. The absence or paucity 

 of Fiicus and Laminaria may give the vegetation a very different 

 aspect from that of most northern seas. In parts of the Mediter- 

 ranean where the high temperatures of the air and absence of appreci- 

 able tides result in reduction of the littoral vegetation, Eel-grasses 

 (Zostera spp.) may grow densely in the upper sublittoral where the 

 bottom is muddy. They are accompanied by epiphytic and other 

 Algae, while near-by sandy bottoms may be largely covered with 

 ' meadows ' of Posidonia oceanica, also a Monocotyledon, as deep 

 down as 60 metres. At 80-100 metres only isolated plants of the 

 Posidonia occur, though algal vegetation may still be luxuriant at 

 a depth of 120-130 metres, and, in exceptionally clear water, fair 

 numbers of Algae may persist to at least 180 metres, or, in some 

 cases, to 200 metres. 



In rocky places in the Mediterranean the supralittoral zone is 

 characterized by a dark girdle of the Lichen Verrucaria matira, or of 

 various Cyanophyceae where the substratum is calcareous, while 

 eulittoral rocks may support a fine girdle of Rissoella verruculosa 

 which, though a Red Alga, somewhat resembles the brown Wracks 

 of more northerly shores. The lithophytic vegetation of the sub- 

 littoral zones is rather various and rich in different forms, showing 

 distinctions occasioned by differences in illumination— chiefly at 

 different depths but also to some extent according to shadows cast 

 by irregularities of the coast. Thus the shade-species predominating 

 in the depths are chiefly Red Algae, such as species oi Lithothamnium, 

 while the Brown Algae, such as species of Cystoseira, prefer brighter 

 areas, and some of the Green Algae, such as Acetahidaria acetahula 

 {A. mediterranea), favour the very brightest spots. The Red Algae 

 of well-lit places are usually dull in colour ; at the other extreme 

 are species so sensitive to light that they are restricted to shaded 

 areas even at considerable depths. In any case the active vegetative 

 season in the Mediterranean largely coincides, near the surface, with 

 the winter and spring, whereas in deeper water the chief activity 

 occurs in the summer and autumn. During early summer, Brown 

 Algae may even prevail over Red Algae in deep water, whereas at 



